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Coming to a Parking Lot Near You: Fast-Draining Concrete

68 LastCall.1 400A new and improved variety of permeable pavement developed in the U.K. called Topmix Permeable has been turning heads.

This new concrete, from Lafarge Tarmac, could potentially be a very useful tool in combating urban flash flooding from sudden, heavy storms – the type that are likely to become increasingly common because of climate change.

In the U.S., stormwater routinely overwhelms antiquated wastewater systems, causing untreated sewage to be dumped into local waterways and polluting public beaches. Storm runoff is also a significant source of pollution for rivers, streams, and reservoirs around the globe. Topmix pavement filters pollutants such as motor oil residue, even as it allows water to drain into the ground below.

A permeable design encompasses not just a structural design but also a hydraulic one. This design is based on many factors such as the design storm event ground permeability and total area to be drained from. This would then normally determine the required thickness of the attenuation layer. (This is basically a storage layer made up of an aggregate with 30 to 40 percent voids). It is in this layer that the water would sit, not within the structure of the permeable surface layer so if the water was to freeze it would not have any detrimental effect on this.

68 LastCall.2 400Topmix Permeable is a fast-draining concrete pavement solution that rapidly directs stormwater off streets, parking surfaces, driveways and walkways. This minimizes the cost and long-term maintenance for local authorities and developers of stormwater management.

Topmix Permeable has an average permeability rate of 36,000 mm per hour. The minimum required permeability rates will vary greatly according to local climate conditions, but as an approximate guide, the average permeability required to cope with a 100-year storm event is 300 mm per hour.

As the permeability rate of this material is so great, there would have to be an unrealistic amount of dirt applied to the surface for the pavement to cease functioning effectively. It is recommended that permeable paving must still be able to function effectively when blocked by up to 95 percent.

Topmix Permeable is not recommended for use in areas which are going to be used for activities such as stockpiling sawdust or in areas where there will be heavy silt loads such as large recycling centers, but is suitable for most other applications.